# Strings library

From cppreference.com
< cpp

### Characters

In the C++ standard library, a character is an object which, when treated sequentially, can represent text.

The term means not only objects of character types, but also any value that can be represented by a type that provides the definitions specified in the strings library and following libraries:

- localization library
- input/output library

regular expressions library | (since C++11)

In the strings library and regular expressions library(since C++11), a character can be of only char-like types, i.e. those non-array types that satisfy the requirements of PODType(until C++20)TrivialType and StandardLayoutType(since C++20)(until C++26)TriviallyCopyable and StandardLayoutType(since C++26).

For any char-like type `T`, std::is_trivially_default_constructible_v<T> is true. | (since C++26)

Therefore, characters are also referred as char-like objects in the strings library and regular expressions library(since C++11).

Some standard library components accept character container types. They, too, are types used to represent individual characters. Such types are used for one of the template arguments of std::char_traits and the class templates which use std::char_traits.

### Library components

The C++ strings library includes the following components:

#### Character traits

Many character-related class templates (such as std::basic_string) need a set of related types and functions to complete the definition of their semantics. These types and functions are provided as a set of member typedef names and functions in the template parameter `Traits` used by each such template. The classes which are able to complete those semantics are CharTraits.

The string library provides the class template std::char_traits that defines types and functions for std::basic_string and std::basic_string_view(since C++17).

The following specializations are defined, all of them satisfy the CharTraits requirements:

Defined in header `<string>` |  |
template<> class char_traits<char>; |  |
template<> class char_traits<wchar_t>; |  |
template<> class char_traits<char8_t>; |  | (since C++20)
template<> class char_traits<char16_t>; |  | (since C++11)
template<> class char_traits<char32_t>; |  | (since C++11)

When a user-defined character container type for std::basic_string and std::basic_string_view(since C++17) is used, it is also necessary to provide a corresponding character trait class (which can be a specialization of std::char_traits).

#### String classes (std::string etc.)

The class template std::basic_string generalizes how sequences of characters are manipulated and stored. String creation, manipulation, and destruction are all handled by a convenient set of class methods and related functions.

Several specializations of std::basic_string are provided for commonly-used types:

Defined in header `<string>`
Type | Definition
`std::string` | std::basic_string<char>
`std::wstring` | std::basic_string<wchar_t>
`std::u8string` (since C++20) | std::basic_string<char8_t>
`std::u16string` (since C++11) | std::basic_string<char16_t>
`std::u32string` (since C++11) | std::basic_string<char32_t>

#### String view classes (std::string_view etc.) (since C++17)

The class template std::basic_string_view provides a lightweight object that offers read-only access to a string or a part of a string using an interface similar to the interface of std::basic_string.

Several specializations of std::basic_string_view are provided for commonly-used types:

Defined in header `<string_view>`
Type | Definition
`std::string_view` | std::basic_string_view<char>
`std::wstring_view` | std::basic_string_view<wchar_t>
`std::u8string_view` (since C++20) | std::basic_string_view<char8_t>
`std::u16string_view` | std::basic_string_view<char16_t>
`std::u32string_view` | std::basic_string_view<char32_t>

### Relevant libraries

The text processing library provides support for localizations, string conversions (e.g. `std::toupper`), character classification functions (e.g. `std::isspace`), and text encoding recognition (`std::text_encoding`).

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 1170 | C++98 | char-like types could be array types | prohibited

### See also

C documentation for Strings library

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